Any preferences for an input cap? I'm using a 0.1uF Russian silver mica. I also have the Russian teflon ones of various kinds I could try - thinking FT-2.

Andy

The Russian Telflons (фтоуропласт) are certainly worth trying, I use the К72П-6 56nF/500V which are easy to find. The 1.25 inch pipe cutter from Rapid Electronics can fetch the steel case off them, when you get the hang of it (buy an extra cap to practice)

My other favourite is a sleeper - the LCR components PC/HV/S high quality industrial polypropylene. For coupling, there may be a better cap, but I have not found one. 1kV or 1.5kV rating, too. These can be used from preamps all the way up to Tesla coils - they are so well built.

I tried out two different supplies to your boards - a Thurby Thandar bench supply and a choke input supply. Both sound different - still experimenting.

What's your guess at the best supply to precede your boards? Seems like differences are audible.

Before drawing conclusions, it is best to get an understanding what your implementations of the PSUs actually do. Choke input is not just equal to any choke input. Hook up a scope and check AC voltages across the secondary of the transformer and right after the rectifier bridge. Especially with choke input you can have some nasty spikes there which can creep into the circuit. Sometimes it is a good idea to tame these with a small cap in front of the choke. Try something like 47uF and move upwards until the spikes with rough edges dissappear.

Before drawing conclusions, it is best to get an understanding what your implementations of the PSUs actually do. Choke input is not just equal to any choke input. Hook up a scope and check AC voltages across the secondary of the transformer and right after the rectifier bridge. Especially with choke input you can have some nasty spikes there which can creep into the circuit. Sometimes it is a good idea to tame these with a small cap in front of the choke. Try something like 47uF and move upwards until the spikes with rough edges dissappear.

Best regards ... Thomas

Andy, this is good advice from Thomas.

Choke input means a big reduction in the peak current into the reservoir caps. Reducing this current can improve the sound of other parts of the audio system connected to the same mains supply.

The type of mains trafo is also important. split bobbin type reduces noise transfer from the mains (compared to toroidal or overwound types), and high quality trafos with an Electrostatic screen can be even better still. My preference is to use an isolating transformer, and an HF barrier in the safety earth (Sadly, safety earth can transmit some troublesome noise): the drawing shows a suggestion for connecting it.

What did you think of grid bias with battery, before settling on filament bias?

I never liked battery bias. And I also never 'settled' on filament bias

In most of my designs I use regular cathode bias with the ultrapath topology. I only use filament bias if all other aspects in the design are already on the best possible level and when the heat generated in the filament bias resistor is of no concern. While the latter is not much of an issue in a 26 stage, the heat dissipation can be excessive with 801As.